Tumors are dependent upon new blood vessel formation, or angiogenesis, for expansive growth. Our recent analysis of gene expression led to the identification of several genes in endothelial cells that line tumor blood vessels. The products of these genes are of particular interest because they reside on the cell surface and may therefore be directly accessible to blood-borne therapeutics. In an attempt to understand the functional role of one of these genes, namely CD276, in tumor angiogenesis, our laboratory has generated a CD276 conditional knockout mice. By challenging gene disrupted mice with tumors, we hope to determine if CD276 is critical for tumor angiogenesis. The studies should also allow us to better understand the normal physiological function of CD276.